OSGi once stood for "Open Services Gateway initiative", but that name slid from "impossibly vague" to "entirely obsolete" rather quickly. For our needs, OSGi is a mechanism for bringing sanity to the "big pile of Jars" that you might otherwise have in a large Java system. It provides a standardized way to describe the name of a library, its version, its dependencies, its capabilities, and its interactions with other libraries. In this way, rather than just "commons-io-2.1.jar", you can conceptually deal with "I require org.apache.commons.io, version 2.0 or above" and the environment can suss out what that means based on what is installed.
As Domino developers, we care about a subset of the OSGi concepts, primarily those related to creating and loading plugins. There is a handful of vital terms involved, some of which I've already been tossing around:
- JAR: a Java Archive file is a ZIP file aimed at Java, generally containing classes and other resources (text files, images, and so forth). There are variants like "War" (Web Application Archive) and "Ear" (Enterprise Archive), but they're all still ZIP files.
- Bundle: A "bundle" is, in effect, OSGi's word for a JAR. It expands on a contained file called
META-INF/MANIFEST.MF (so named because Java likes to be difficult sometimes) to define the description of the bundle: its name, version, and so forth.
- Plug-in: This is basically the same thing as a bundle. Presumably, it technically refers to a specialized kind of bundle, but we can use the terms interchangeably. It also technically has that hyphen in it, but I often write it "plugin" anyway.
- Feature: This excessively-vague term is OSGi's way of grouping plugins together into a single conceptual installable unit. For example, the main feature included in the Extension Library references ten distinct plugins.
- Update Site: Update sites are to features what features are to plugins: a way to organize these features into a collection, either to install them all at once or to provide a pool of available software to install. Eclipse-the-IDE uses them extensively for the latter case. As Domino developers, we primarily use them to distribute libraries and install them into the server's Update Site NSF.
- Target Platform: A target platform refers to the set of plugins available in a given environment, which is used to determine whether a given plugin has the dependencies it will need to run. In our case, the target platform is usually the XPages runtime environment, containing OSGi base plugins, Java servlet support plugins, and the XPages runtime plugins themselves (among others). Plugin development environments use these heavily.
- Extension Point: An extension point is a way for a plugin to either declare that it can either provide or consume a given service. For example, you might have a plugin that says "I can make use of any plugin that provides a color" and then another that says "I can provide the color blue". By using extension points, the first plugin can find the other, even though it may have been developed entirely independently.
- Java EE: Java Enterprise Edition (sometimes written J2EE thanks to Java's weird relationship with versions) is basically the standard "web server stuff" for Domino (though it covers other aspects). XPages is sort of like a Java EE environment, but the experience is distinct. Notably, a Java EE server is a general platform on which other web-dev toolkits - JSP, JSF, Vaadin, JAX-RS, and so forth - can run. Java EE isn't inherently related to OSGi, but the two eventually bleed into each other with Domino and WebSphere.
OSGi in general goes beyond that, and some other aspects may be useful down the line (such as interacting with the OSGi console), but for now it's enough to think of it as a set of decorations for your projects. With some of the basic terms in line, next I will move on to setting up an Eclipse environment to start development.